Combining supplies

[dobo99]

Hi, been asked to tender for a hydroelectric project. As part of it have been asked to combine the 2 supplies coming back to the main switch room. 1 supply is from a 30Kw turbine 500m away and the other is a 90Kw turbine 50m away. Both supplies have to join to a 185mm swa that is already in the switch room.

Any ideas? I know what I'm thinking but wanted other ideas

 

[Tony S]

I assume you know about synchronising?

Are the alternators connected directly to the common bus or via isolation transformers? Direct to bus can give rise to circulating currents due to impedance imbalance and phase overlap.

The more information you can give the better the answer.

 

[dobo99]

We've only been asked to quote for the LV side of it, there is a specialist turbine company involved as well as a HV contractor as due to distances involved the 90Kw supply is to be stepped up for transmission back to the switch room then a 2nd transformer to drop back to LV. All it says in the performance spec is to combine the 2 supplies onto an existing 185mm swa 

 

[Rob.]

I'm afraid until we get some more information we can't really advise the best route to take here. 

As Tony has said, simple differences in design can end up causing issues doing it one way or another. 

Also do you really need a HV supply for a 30kW 500 meter run? The control gear an transformers surely must cos more than just running a couple of decent sized cables in parallel. I mean its hardly a substantial load.

 

[Tony S]

Isolating transformers for each alternator would simplify the design by negating incompatibility. As for the step up/down transformers, I agree with RoB, parallel cables would be cheaper.

 

[Tony S]

I’ve e-mailed a copy of the drawing to RoB, it can’t be posted here due to sensitive information.

What exactly have you been asked to quote for.

 

[Tony S]

I’ve looked at the drawing again and the step up/down transformers now make sense. At 3.3kV a single 35mm² would be better and cheaper than 2 x 185mm².

 

[Rob.]

Yes 90KkW over 950m would probably benefit it. 

The description the OP gave and the schematic show different cable runs.